TREE ROOTS, WATER UPTAKE, AND IMPLICATIONS FOR SOIL PRODUCTION THROUGH TIME IN NORTHWEST ARKANSAS

How tree roots contribute to physical weathering processes and thus soil generation remains an open question. Evidence suggests roots may damage rock by accessing fractures, where they may expand due to water uptake or generate forces on rock in response to wind gusts. I monitored forces at the root-rock interface for an American elm and Hackberry tree between September 2019 and May 2020 in a temperate karst setting in NW Arkansas. I used piezoelectric force sensors to determine if differences in species, tree size, distance of roots from the tree, wind or precipitation conditions affected the frequency or magnitude of forces exerted by tree roots onto bedrock. I analyzed meteorological conditions in addition to root forces to examine environmental controls on diurnal force cycles exerted on bedrock. Additionally, I quantified how roots responded to wind gusts and rainfall events. Roots of both species exerted higher forces between the hours of 10:30 and 23:00, peaking between 15:00 and 18:00, and exerting forces for an extra hour during the fall and spring compared to the winter. My data suggests that temperature’s impact on vapor pressure deficit, which controls the rate of transpiration, was the primary driver of the timing of daily forces. Precipitation led to periods of higher forces, as the roots expanded due to water-uptake as well as reduced tree transpiration from lower VPD and solar radiation during rainfall events. Roots of greater size exerted more fluctuations in forces onto the bedrock in response to wind gusts and rainfall. American elm roots exerted forces on the bedrock more frequently during windy periods compared to the Hackberry roots. Variations in the root response to wind and precipitation events are hypothesized to be linked to contrasting rooting strategies between species and the functional role of individual roots in supporting the tree. Based on an analysis of climate data over the last 40 years, my findings suggest that in warmer conditions, with more intense rainfall events, roots will exert greater forces on bedrock due to increased temperature-controlled vapor pressure deficit and, heavy rainfall-induced forces due to water uptake. These projected increases in forces suggest that in the karst landscape of Northwest Arkansas, tree roots may accelerate the physical weathering of bedrock.